Dibenzylidene sorbitol derivatives or dibenzylidene xylitol derivatives even used in a small amount can gel a wide range of organic solvents or polymers (Kobayashi et al Journal of Japan Rheology Society, vol. 17, p.104 (1989) and Kobayashi et al, Journal of Japan Rheology Society, vol. 17, p.112 (1989)). These derivatives are used in a wide variety of applications, chiefly as solid marking agents, solid adhesives, effluent oil gelling agents, sol-gel transition type clarifying nucleating agents for polyolefins and so on. However, said derivatives are sparingly dissolved in water and can not be used as a gelling agent alone for water.
Various high molecular weight gelling agents useful in forming a heat-reversible water-containing gel are known and include non-electrolytic polymers such as natural high molecular weight starch or cellulose derivatives, electrolytic polymers such as alginic acid, agar and carrageenan, and protein-containing polymers such as gelatin, collagen and casein. Synthetic polymers such as copolymers of poval and sodium polyacrylate, semi-synthetic polymers such as starch-acrylic acid graft polymers, and the like are widely used as a water-absorbing polymer in the manufacture of paper diapers. However, natural materials have a drawback of decomposing. The gel-forming ability of electrolytic polymers is seriously affected by a salt and is markedly impaired in the presence of a salt. These polymers pose a problem of operational efficiency because they are dissolved in water usually at a low rate and a sol system is given an elevated viscosity.
Sericin, which is expensive and a specific species of amino acid, has an ability to gel water, but its gelling ability is markedly affected by the acidity of a system. Further, sericin involves a problem in terms of preservation. For these reasons, sericin has not been put to practical use.
When an aqueous medium is gelled using a conventional low molecular weight gelling agent, typically a dibenzylidene sorbitol derivative, an organic solvent is essentially used in combination.
In the above situation, there is an earnest desire for neutral low molecular weight organic gelling agents free from said defects, namely, those which can be stored without decomposition, do not adversely affect the environment, show high efficiency in a gel-forming process from a stage of sol formed on dissolution and are not affected by the presence of an inorganic salt.